The home entertainment industry is one of the fastest growing areas of the world economy. Direct Satellite Service television, the explosive growth of cable television channels, video on demand, and a host of other services have changed the ordinary television set into a home entertainment center, opening the door to an enormous amount of variety and options. Furthermore, the Internet, the World Wide Web, and the incredible assortment of on-line services, have likewise immersed the home computer in a sea of information. Combining the two--the television as a center for viewing video with the data storage and on-line capabilities of the home computer--is a challenging task that is not without difficulties. The independent developments of these areas of technology have produced sometimes conflicting standards, a natural consequence of the divergent origin of computer and entertainment media.
Television monitors typically present video images in the form of a rapid sequence of video fields, changed at a high frequency to create the illusion of motion. Television cameras and other sources of video generally do not produce full-frame images; rather, such video sources typically produce a field consisting of about half of the lines of each full-frame image, at a rate of 60 such fields per second. Alternate fields contain alternate lines of video data. In other words, one field contains the odd-numbered lines, and the next field contains the even-numbered lines. Accordingly, each field of the video may be identified as an "odd" field or an "even" field. The sequence of video fields alternates between the odd fields and the even fields. A television monitor receiving the sequence of fields then reproduces each video field in the sequence. Each field is displayed on the television screen only on half of the scan lines; first an odd field is displayed, using the odd-numbered scan lines, then an even-field is displayed using the even-numbered field lines, etc. The television scans a raster across the screen from the top left to the top right, producing a first scan line ("scan line #1"), then invisibly returning the raster to the left edge of the screen to a position slightly below the original position. The position to which the raster returns, however, is not immediately below the first scan line, but allows sufficient space to accommodate an intervening scan line on the alternate field. The raster then scans across to the right edge of the screen to produce a second scan line ("scan line #3"), and thus continuing to the bottom edge of the screen. The distance between the scan lines is a function of the size of the window, but generally allows an intervening scan line (the first scan line of the other field, i.e. "scan line #2") to be drawn after the completion of the first field. The invisible return of the raster to the left edge of the screen after scanning each scan line is a flyback or horizontal refresh stage that occurs much more rapidly than the visible left-to-right lines. In this manner, approximately 485 active scan lines may be produced to complete a single video frame, half of which is displayed in each field. Once reaching the bottom edge of the screen, the raster is then invisibly returned to the original position at the top left corner during a "vertical blanking interval" stage. The horizontal and vertical blanking interval stages are high speed and invisible. Sixty fields per second may be produced. With respect to typical television, this "interlaced" video scanning approach is an appropriate compromise between vertical refresh rate, vertical resolution, and limited bandwidth.
Although alternating between an odd frame and an even frame may be appropriate for lower -resolution real time video images, computer displays generally must use a progressive video system in which each full frame is presented in sequence, in order to reproduce precise higher resolution graphics information. Displaying computer information in an interlaced manner, in order to mix it with video, would present several unacceptable artifacts. For example, when a narrow horizontal line or edge is a significant part of the image, the line may appear in only one of the two fields. If the narrow horizontal line happens to fall in the odd field, that is, on a scan line having an odd number, then the line only appears when the odd field is being presented. Likewise, a horizontal line in the even field would only be presented when the even field is being presented. Because the odd and even fields alternate, the line or edge thus appears to blink or flicker at a rate of 30 times per second, which is noticeable to most people. This effect is illustrated in FIGS. 5A-5C, discussed below. A similar problem occurs when a horizontal lines is two pixels in width. If a horizontal line is two pixels in width, a single line will appear in each of the odd and even fields. Because these fields are presented alternately, the horizontal line will appear to move slightly in the vertical direction, apparently bouncing or wavering. A two-pixel wide line is shown in FIG. 5D. Not only are these artifacts of flicker and waver distracting in computer graphics, where precise location of lines and edges is crucial, but are exacerbated in a large-screen display, in which each scan line or pixel of the video image is translated to a large region of color or of gray scale on the large screen, making waver even more noticeable.
This leaves the approach of trying to display the interlaced video in a progressive manner, in order to mix the two sources. One approach for this is the "static mesh" approach, in which two successive fields of video are combined and displayed simultaneously. Referring to FIGS. 4A and 4B, the static mesh approach is shown, illustrating deinterlacing of the odd and even fields when both fields contain data. The scan lines in the odd field are mapped to odd lines on the display, and the scan lines in the even field are mapped to the even lines on the display. While this static mesh approach works ideally for stationary images, distracting artifacts are created when the image is moving, since the odd and even fields are received at different times and describe the image (as seen at the video source) at slightly different moments in time. Typically, the temporal displacement between successive fields is 1/60th of a second. When an image moves, successive video fields present the image in a slightly different position. If the object moves slowly, and the odd field and the even field are shown together in the same frame, any edge becomes blurred or shadowed, as the two images appear close to one another. Another problem caused by the motion of the object is double-vision or ghosting, seen when the object is moving more quickly. One effect of the motion of the image is shown in FIG. 4C. Two images may be presented, as the odd lines of the image show one field and the even lines of the image show a different field. Because the even and odd fields are from different times, combining the two images in the "static mesh" can produce a "tearing" or "feathering" around the edges of moving items. This effect is highly noticeable to many users. Although the static mesh approach is highly appropriate to still images, this tearing or feathering approach is noticeable when an image is moving.